Difference in WordPress comments: reply to existing versus adding a new comment?

I'm having a hard time finding how to differentiate in the WordPress comment forms when you are replying to an e开发者_高级运维xisting comment (threaded) versus replying the the blog post, and not another comment.

Both forms seem to have the same action and even same hidden value. How does the form post associate the new comment as either a threaded reply, or just a parent-level reply?

I tried searching WordPress' support but didn't have the right vocabulary/search terms to get a valid search result.

If you check database of wordpress, you can see there is a table named 'wp_comments'. In that table there is a column 'comment_parent' which holds the id of parent of comment. If its value is 0, it means it is comment to post and if its value is non-zero, it means it is reply to that prent comment